How much time did the 9yo spend alone in his room that he would have had time to rifle through things? My boys are always insisting things are in places that they are not, they'd bet their lives on it yet, the item will be found elsewhere and this has happened with $ a lot as well.
Even if you ask the boy if he took it, what's he going to say? You can't prove he did it and did he really have the opportunity? I can see if maybe it was sitting out in the open but hidden away in a drawer?